Fully understanding a capacitor circuit

In summary: As the capacitor charges, the difference in voltage between the plates increases. This difference in voltage is called an EMF (electromotive force).As the capacitor charges, the EMF will keep trying to drive current through the resistor. However, because the resistor resists the flow of current, the EMF eventually dissipates and the current stops. At this point, the voltage across the resistor is the same as the voltage across the capacitor. This is called the "low-pass" or "resistance" limit of the circuit.
  • #1
mklein
43
0
Dear forum users

Firstly, I wanted to post in the General Physics area, but it wouldn't let me create a new thread there!

I am a secondary school science teacher, teaching physics up to A-level.

I was in the process of revising capacitors with the class and it occurred to me that I may have a bit of a gap in my understanding.

Consider a simple series circuit with a cell, capacitor and resistor. While charging, charge collects on the capacitor plates until the voltage over the capacitor equals the voltage of the cell. While a current flows there can be a voltage over the resistor too.

When fully charged, the voltage of the cell will equal that of the capacitor. But there will be no voltage over the resistor.

Of course I am happy with the result, but I am having trouble explaining exactly WHY there is only a voltage over the resistor while a current flows – and then there is no voltage over it when the current stops.

Does this have something to do with the difference in meaning between an EMF and a voltage? i.e. the cell and capacitor create an EMF (like a ‘push’ due to static charge on the plates) whereas with the resistor just experiences a loss of potential due to resistance?

Any explanation of this result would be really appreciated

Matt Klein
 
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  • #2
Welcome to the Forum!

The reason a voltage appears across the resistor has to do with forcing current flow. A voltage drop can be thought of as an energy potential difference needed to drive current through the resistor, which, aptly enough, resists the flow. Sometimes an analogy to water flowing in a pipe is helpful. A small pipe "resists" flow through friction with its walls. To produce a flow, you must drive the water through with a pressure differential across the pipe.

Here is a useful link:
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html"

Click on "Ohms Law", "Resistance" and "Resistivity" on the index at right, and explore the further links on those pages.
 
Last edited by a moderator:
  • #3
Welcome Matt,

Don't forget Ohm's law:

V = I R

If I = 0 then V = 0, you cannot have one without the other.
 
  • #4
The way I see it is I imagine the capacitor with just two plates for simplicity. When the capacitor is charged, the plates are at the same voltage. Thus, there is no difference in voltage between the plates and so the charges don't flow. No flow of charge means no current.
 

1. What is a capacitor circuit?

A capacitor circuit is a combination of capacitors, resistors, and other electrical components that are connected together to store and release electrical energy. It is used in various electronic devices to block DC currents, store energy, and filter out unwanted signals.

2. How does a capacitor circuit work?

A capacitor circuit works by storing electrical charge in the form of an electric field between two conductive plates separated by an insulating material called a dielectric. When a potential difference is applied to the capacitor, electrons accumulate on one plate, while the other plate becomes positively charged. This creates an electric field that stores energy.

3. What is the role of capacitors in a circuit?

Capacitors play a crucial role in a circuit by storing and releasing electrical energy. They act as temporary energy storage devices and can also block DC currents while allowing AC currents to pass through. They are also used for power supply filtering, signal coupling, and timing circuits.

4. How do you calculate the capacitance of a capacitor circuit?

The capacitance of a capacitor circuit can be calculated by using the formula C = Q/V, where C is the capacitance in farads, Q is the charge stored on the capacitor in coulombs, and V is the potential difference across the capacitor in volts. The capacitance can also be determined by the physical characteristics of the capacitor, such as the distance between the plates and the surface area of the plates.

5. How can I fully understand a capacitor circuit?

Fully understanding a capacitor circuit requires knowledge of basic electrical concepts, such as voltage, current, and resistance, as well as an understanding of how capacitors work. It is important to study and practice with different capacitor circuit configurations and to familiarize yourself with the various equations and calculations involved. Additionally, seeking guidance from experienced professionals and experimenting with different circuit designs can help deepen your understanding.

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